The invention generally relates to preconditioning membranes of a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).
The membranes of a newly assembled fuel cell stack typically are cycled through an incubation period, a period of stack operation to xe2x80x9cbreak-inxe2x80x9d the membranes. Until the membranes are broken in, the terminal voltage of the stack gradually rises over time before the terminal voltage stabilizes near a generally constant voltage level to mark the end of the incubation period. Among the possible theories to explain why the incubation period is needed, the membranes may include catalyst residue that, until removed during the incubation period, hinders the performance of the membranes. Another theory is that the membranes are initially dry, a condition that hinders the performance of the stack until the membranes hydrate during the incubation period.
Referring to FIG. 1, during a typical incubation period (represented by a time interval called TINC in FIG. 1), a load is placed on the stack, and humidified oxidant and fuel flows are provided to the stack to produce a terminal voltage (called VTERM in FIG. 1). As the incubation period elapses, the VTERM terminal voltage gradually rises. During the incubation period, the load may be varied or may be kept constant. Eventually, the rate at which the VTERM voltage changes decreases, and the VTERM voltage stays near a relatively constant voltage level (called Vl in FIG. 1), thereby marking the end of the incubation period and the beginning of the useful life of the stack. It is noted that the VTERM voltage may gradually decrease away from the Vl voltage level over the lifetime of the stack.
The incubation period may take approximately two to four hours, a time interval that is a significant component of the overall time that is needed to manufacture the fuel cell system. Thus, the incubation period may have a significant impact on the overall cost of the fuel cell system. Therefore, there is a continuing need for an arrangement and/or technique to reduce the time needed to incubate the membranes of the fuel cell stack.
In an embodiment of the invention, a technique is used to reduce the time that is needed to incubate a membrane of a fuel cell. The technique includes providing gas flows to an anode region and a cathode region of the fuel cell during a first time interval without causing electrochemical reactions to occur in the fuel cell. Subsequently, during a second time interval, the technique includes causing electrochemical reactions to occur in the fuel cell to incubate the membrane.